Tertiary alkyl ethers, particularly methyl tert-butyl ether (MTBE) and tert-amyl methyl ether (TAME), are of high interest for improving gasoline qualities, particularly in view of their anti-knock properties.
It is known to manufacture tertiary alkyl ethers, particularly methyl tert-butyl ether (MTBE) and tert-amyl methyl ether (TAME) which are the most conventional ethers, by reacting an iso-olefin, generally contained in a hydrocarbon fraction, with an aliphatic alcohol, for example methanol, in the presence of an acid catalyst, for example sulfuric acid, hydrofluoric acid, aluminum chloride or boron fluoride, or in the presence of carbonaceous materials containing -SO.sub.3 H, for example sulfonated coals, sulfonated phenol-formaldehyde resins. sulfonated coumarone-idene polymers or preferably sulfonated polystyrene-divinylbenzene resins.
It has been known for a long time that the reaction between methanol and tertiary olefins is a balanced reaction, and therefore it is difficult to obtain acceptable conversion rates of iso-olefins without using a very high methanol excess with respect to the stoichiometrical amount; however, methanol forms azeotropes with the light hydrocarbons complicating its recovery by conventional methods such as, for example, azeotropic distillation combined with recycling to the reactor, as disclosed in the French patent 2 411 881. U.S. Pat. No. 4,204,077 teaches methanol recovery by extraction with a solvent such as ethylene glycol. However these methods require costly investment and are complex in operation (due to the necessary use of several reactors and distillation columns to produce and separate the tertiary alkyl ether).
A method has thus been proposed for converting a major part of the iso-olefins without necessarily using a methanol excess : it consists of the reactive distillation (or catalytic distillation) method, according to which the etherification reaction with the catalyst and the distillation for separating the tertiary alkyl ether, as it is formed, from the other unconverted constituents, are performed in the same enclosure (U.S. Pat. No. 3,629,478, EP-B 8 860, FR 2 503 700).
The European patent 8 860 proposes to use a distillation column, filled with a catalyst convenient for producing methyl tert-butyl ether (MTBE) wherein the catalyst also acts as packing for the distillation, thus forming MTBE and simultaneously separating C.sub.4 constituents. Although the process disclosed in this patent represents substantial progress in the field of reactive distillation, the contact between the liquid phase and the catalyst seems to be more or less intermittent, due to the disturbing effect of the vapor phase.
French patent 2 503 700 proposes the use of a series of catalytic steps with ascending vapor-liquid flow through each catalyst bed with the catalyst being embedded. But the distillation effect is not as important as expected. Moreover, a hydrodynamic problem may arise : As a matter of fact, in view of the gravity effect, it will not be easy for the liquid to flow upwardly through each catalyst bed.
U.S. Pat. No. 3,629,478, which represents the closest prior art, discloses the use of distillation trays and the placement of the catalyst in bulk (i.e., freely) only in the downcomers of said distillation trays, in order to avoid the disturbing effect of the vapor phase through the catalyst. However, the presence of catalyst in such downcomers generates such a pressure drop that the liquid preferably flows down counter-currently through the orifices provided for the vapor on the working table of each distillation tray, instead of passing through the catalyst bed which is thus bypassed.